This invention relates to new methods and apparatus for advanced video systems, including advanced television systems.
The original monochrome television broadcasting standard for the United States was set in 1941 by the National Television Systems Committee (NTSC). The color television standard was established in 1953 by the NTSC in a manner compatible with the monochrome standard. This NTSC standard calls for 525 lines interlaced at 30 frames/second, with each frame represented by two interlaced fields at a rate of 60 fields/second, and an overall channel bandwidth of 6 MHz. More than 15% of the transmission time is reserved for blanking (retrace time) to allow the display to be scanned in synchronism with the camera.
In the NTSC system, each frame is scanned or decomposed into two related fields, one field is raster scanned along the odd number lines and then the second field is scanned along the even number lines. This field information is broadcast and the receiver displays the fields in an interlaced manner.
Various types of program material can be broadcast under the NTSC standard. Conventional moving picture films are a common source of program material. For viewing these films in theatres, 24 film frames (scenes) are used in one second; to avoid flicker, however, each film frame is shown twice. It is therefore necessary to convert 24 frames per second film to the NTSC video rate of 60 fields/second, such as by the known 2:3 pull-down method. In this method, one film frame is converted to two fields (one odd field and one even field) and the next (second) film frame is converted to three fields (one odd field, one even field, and one odd field). The next (third) film frame is converted to two fields (one even field and one odd field), and the next (fourth) film frame is converted to three fields (one even field, one odd field, and one even field). The process continues in this manner, creating 6 sets of 10 fields for every set of 24 film frames, thus converting the film into the 60 fields per second rate required by NTSC. SECAM and PAL, two other standards used in various parts of the world including Europe, operate at 50 fields per second and normally employ a known 24/50 conversion technique for converting film to video.
In the case of program material not recorded on film, the conventional camera scans the odd field and then the even field 30 times per second, thus creating the 60 fields per second rate required by NTSC; SECAM and
scan the two fields at 25 times per second, thus creating 50 fields per second.